RecurlyPushListenerController doesn't pass client object when calling \Recurly_Account::get

I added a commit to the 4.x branch that fixes the bug so that push notifications can actually be processed now. However, I also noticed there are no tests for push notifications so it would be good to add some tests too.

Here's an MR that adds tests for the push notification controller. I had to make one change to the code in the controller, switching from `file_get_contents('php://input')` to `$request->getContent()` to make it easier to mock the request. And, this is probably a more appropriate use of the Drupal request API anyway. This doesn't change anything about how this works for anyone's current implementations. Just adds tests.

Automatically closed - issue fixed for 2 weeks with no activity.